1. Field of the Invention
The present invention relates to an inkjet recording apparatus and an ink supply method, and more particularly to technology for controlling the back pressure of ink supplied to an inkjet head.
2. Description of the Related Art
An inkjet recording apparatus (inkjet printer) as an image forming apparatus has been known which includes an inkjet head (print head) having a plurality of nozzles (ink ejection ports) and which forms an image on a recording medium by ejecting ink in the form of liquid droplets from the nozzles while causing the inkjet head and the recording medium to move relatively with respect to each other.
In this inkjet recording apparatus, the ink is typically supplied from an ink tank which stores ink, to the inkjet head, via an ink supply channel. During printing, the nozzles of the inkjet head are required to be filled with ink at all times, in order to be able to carry out printing immediately whenever there is a print instruction, but on the other hand, it is also necessary to keep the ink pressure inside the nozzles at a negative pressure, in order to prevent the ink from leaking out from the nozzles.
Therefore, various methods have been proposed for controlling the back pressure in the inkjet head in such a manner that the nozzle sections in the head assume a negative pressure.
For example, Japanese Patent Application Publication No. 2005-041048 discloses a liquid ejection apparatus which includes: a recording head; an ink cartridge which supplies ink to the recording head; and a pressure control unit connected to the ink cartridge via a pressure control tube. The ink cartridge in this liquid ejection apparatus is constituted of an ink case which accommodates one or more ink packs serving as liquid accommodating bags. The pressure control unit in this liquid ejection apparatus is constituted of: a pressure sensor which determines the air pressure inside the pressure control tube; an atmosphere release valve which connects the interior of the pressure control tube to the atmosphere; a pump which controls the air pressure inside the ink cartridge; and a pressure control valve which can open or close the connection between the pressure control tube and the pump. In this liquid ejection apparatus, the negative pressure of the ink in the recording head is controlled by adjusting the air pressure inside the ink cartridge by means of the pump and the pressure control valve so as to keep the air pressure inside the ink cartridge in a prescribed range.
Moreover, Japanese Patent Application Publication No. 2000-141687 discloses an inkjet recording apparatus which includes: a recording head; an ink tank having an ink bag arranged in an external case; a subsidiary tank which has an ink bag and is arranged between the recording head and the ink tank; a pump which applies pressure to each of the ink bags by incorporating air into a space between the external case and the ink bag that is hermetically sealed inside the external case; and a switching valve which switches the connection between the pump, the ink tank and the subsidiary tank (external cases of the ink tank and the subsidiary tank). In this inkjet recording apparatus, the ink is supplied to the subsidiary tank or the ink containing bubbles (gas bubbles) is expelled from the recording head by switching the switching valve so that the ink is supplied to the recording head.
FIG. 11A is a diagram showing the liquid ejection apparatus described in Japanese Patent Application Publication No. 2005-041048 in a case where the liquid ejection apparatus is provided with a plurality of the recording heads 904. As shown in FIG. 11A a cartridge 906 serving as a subsidiary tank is disposed between the main tanks 902 and the recording heads 904, and the cartridge 906 includes a plurality of ink packs 908 that are respectively provided for the main tanks 902 (recording heads 904). The internal pressure Po of the cartridge 906 is adjusted by means of the pump 910 shown in FIG. 11A. However, in this liquid ejection apparatus, it is difficult to control the back pressure independently for each of the recording heads 904, since the levels H1 and H2 of the ink in the ink packs 908 change depending on the remaining amounts of the ink in the ink packs 908 as shown in FIG. 11B.
In this case, in order to control the back pressure in each of the recording heads 904, as shown in FIG. 11C, it is necessary to provide cartridges 906-1 and 906-2 which respectively accommodate ink packs 908-1 and 908-2, separately for each head 904-1 and 904-2. By means of the configuration shown in FIG. 11C, it is possible to control the respective internal pressures P1 and P2, independently, in such a manner that the internal pressure P1 of the ink pack 908-1 having the lower level of the ink is greater than the internal pressure P2 of the ink pack 908-2 having the higher level of the ink, namely a condition of P1>P2 is satisfied (P1 and P2 are negative pressures; and P2 is less than P1). However, if a cartridge is provided for each of the recording heads as shown in FIG. 11C then the size of the liquid ejection apparatus becomes larger.
Moreover, particularly in a case where the liquid ejection apparatus is an inkjet recording apparatus of large ink consumption that includes a full line head which covers the full width of the recording medium and performs recording by means of a single scan, the ink packs are large in size and therefore, in order to make the apparatus as compact as possible, the apparatus is required to extend in a vertical direction (i.e., a direction perpendicular to the recording medium). In this case, the effect of the variation in the pressure head due to the ink consumption on the negative pressure applied to the recording head cannot be ignored, and it is difficult to achieve a uniform back pressure in the plurality of recording heads.
FIG. 12A is a diagram showing a configuration of the liquid ejection apparatus for a single color that includes a main tank 902, a head 904, a cartridge 906, an ink pack 908 and a pump 910. FIG. 12B is a diagram showing a configuration whereby the uniform back pressure is achieved in the plurality of recording heads by providing the configuration (for a single color) shown in FIG. 12A for each of the recording heads 904-1 to 904-3. However, if this configuration is adopted, there is a problem in that the liquid ejection apparatus increases in size and costs also rise.
Furthermore, in order to suppress increase in costs, it would be possible to use a common pump and to provide switching valves which switch the connection between the pump, the ink tanks and the subsidiary tanks (the external cases of the ink tanks and the subsidiary tanks), as in the apparatus described in Japanese Patent Application Publication No. 2000-141687. However, in the case of a switching valve as described in Japanese Patent Application Publication No. 2000-141687, there is a problem in that inks can only be supplied to the recording heads by switching sequentially through the colors, one at a time, and it is difficult to achieve a simultaneous operation for the plurality of colors.
Moreover, since the refilling of ink into the subsidiary tank and the discharge of ink from the subsidiary tank cannot be carried out simultaneously, then in a case where, for example, the consumption of ink of one color is extremely high, the ink supply cannot respond adequately to demand. Further, in order to avoid this problem, it becomes necessary to provide large subsidiary tanks, and therefore the apparatus increases in size and costs are liable to rise.